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16 November 2012

Undulus asperatus

Okay, cloud nerds, it's been a while since I've talked about the great gig in the sky, but I absolutely had to show you this. I was driving through the Atlanta suburbs Wednesday afternoon when I noticed that the sky had this eerie, underwater look to it. I pulled off and took a few photos from a hill beside a parking lot. And got really excited.

The reason I was so excited about this cloud is not only because I think it looks strange and beautiful, but also because it is the center of some hubbub in the world of cloud watchers. Before a few years ago, this cloud did not have a name. But in 2009, the Cloud Appreciation Society started getting enough pictures of this unusual cloud that they decided it needed a name, Undulus asperatus (don't you love how clouds are classified by a Linnean system, just like living things?). Asperatus means agitated, and undulus means waves. Pretty accurate, no?

In the great cloud name game, people universally recognize the World Meteorological Organization as the entity with the power to create new cloud classifications. They publish theInternational Cloud Atlas, basically a field guide for the sky. (Completely unrelated: There is a novel by David Mitchell called Cloud Atlas that was just made into a movie.) The newest cloud in their collection is Cirrus intortus, added in 1951, and the latest edition of the International Cloud Atlas was published in 1975.

As far as why asperatus forms, it is believed that the cloud results from convective storm activity. The wavy underside is the result of warm and cold air meeting at at the boundary of the lower and middle atmospheres (I learned that here.) The analogy often used is oil and vinegar - like the two layers of atmosphere, they don't really mix, and when you shake them up a little with convective activity (moisture, lift, and instability), the result looks something like this:

What do you think? Should it be a new cloud classification? Have you ever seen Undulus asperatus?

Also:
--A picture of Gavin Pretor-Pinney (founder of the Cloud Appreciation Society) at the Royal Meteorological Society. I love this picture because it's exactly what you would imagine a British society devoted to weather to look like - a bunch of men (and one woman) in ties sitting in a room full of gold-framed oil portraits, staring at a table full of cloud pictures.
--The Cloud Appreciation Society's Asperatus page - two stunning images of Asperatus.
--Read all about the classification of clouds here!